Microstructure and Stress Corrosion Cracking of Dissimilar Metal Weld 16MND5/309L/308L/Z2CND18-12N Used for Connecting Reactor Pressure Vessel to Piping in Nuclear Power Plants

Abstract

The microstructure and stress corrosion cracking (SCC) in simulated primary water at 290°C of a dissimilar metal weld 16MND5/309L/308L/Z2CND18-12N made in China were studied for reliability and lifetime assessment and management of relevant pressurized water reactor (PWR) nuclear power plants. Results of microstructure characterization showed that the base metal of the low alloy steel (LAS) 16MND5 was upper-bainitic and stainless steel (SS) Z2CND18-12N was equiaxial austenitic grains, the bulk weld metal of SS 308L was austenite with dendritic structures plus δ-ferrite islands. The interfaces areas around 16MND5/309L and 308L/Z2CND18-12N exhibited apparent fusion lines with complicated microstructure/chemical composition changes, especially the former. The area between the 16MND5 base metal and the bulk weld metal was made up of three parts, that is, the heat-affect zone of 16MND5, fusion line and transition zone. There was a thin martensitic layer along the fusion line, which possessed the highest micro-hardness among the weld. The transition zone was an austenitic layer within which Cr% and Ni% changed significantly. SCC test results showed that this area around 16MND5/309L was the most susceptible to SCC among the weld. Both intergranular and transgranular SCC occurred in this area when tested in at high electrode potential which is mainly related to bad water chemistry with high oxygen contamination. The crack mechanism and the significance in engineering are discussed.